The invention relates to a method for the electrolysis of aqueous solutions of hydrogen chloride in order to produce chlorine by means of gas diffusion electrode while maintaining defined operating parameters.
Aqueous solutions of hydrogen chloride, referred to below as hydrochloric acid, are formed as a waste product in many processes in which organic hydrocarbon compounds are chlorinated in oxidizing fashion with chlorine. The recovery of chlorine from these hydrochloric acids is of economic interest. The recovery can be carried out electrolytically using gas diffusion electrodes which consume oxygen in the cathode space (oxygen-consuming cathode).
A corresponding method is known from U.S. Pat. No. 5,770,035. According to that document, the electrolysis takes place in an electrolysis cell having an anode space with a suitable anode, e.g. a titanium electrode which is doped or coated with precious metal and is filled with the aqueous solution of hydrogen chloride. The chlorine formed at the anode escapes from the anode space and is fed for suitable treatment. The anode space is separated from a cathode space by a commercially available cation exchange membrane. A gas diffusion electrode is positioned on the cation exchange membrane on the cathode side. A current distributor is located behind the gas diffusion electrode. An oxygen-containing gas or pure oxygen is usually introduced into the cathode space.
The nature of the initial operation and normal operation of an electrolysis cell has an influence on the service life of the anodes or of the anode half-element and therefore on the economic viability of the method.
According to U.S. Pat. No. 5,770,035, therefore, an oxidizing agent, for example iron(III) or copper(II) is necessarily added to the solution which is to be electrolyzed in order to protect against corrosion. These additives then have to be removed again from the hydrochloric acid by means of additional outlay of apparatus. Moreover, they contaminate the hydrochloric acid and may under certain circumstances have an adverse effect on the action of the ion exchange membrane or lead to crystallization. U.S. Pat. No. 5,770,035 does not disclose any conditions for initial operation of the cell.
According to conventional methods for initial operation and normal operation, considerable corrosion to the anode coating and to the anode metal, for example titanium, beneath the coating of the anode is inevitable. The anode space, which consists of titanium, is also at risk from corrosion. Corrosion entails high operating costs, a high level of outlay on maintenance and environmental and recycling problems.